NFC is a wireless technology started by Philips, then developed by Nokia, Sony, and other manufacturers, which was originally just a combination of remote control identification and network technology. Now, NFC has become an easy, safe, fast communicational wireless connection technology and has played a great role in access control, public transportation, and mobile payment, etc.
NFC chip(s) built in NFC mobile phones may be used as one part of an RFID module, which may be used as passive RFID tags to pay expenses or as RFID reader-writers for data exchange and acquisition.
In the absence of external interference, data can be transmitted, read and written between the electronic tag antenna and the reader-writer by electromagnetic coupling. However, in the practical applications, for example, there are many metal parts in the mobile phones such as batteries and back covers, which may interfere with signals. The metal parts absorb the electromagnetic waves emitted by the reader-writer and then lose them in the form of eddy current, thereby greatly reducing the electromagnetic waves through the electronic tag antenna and resulting in the reduction in the communication distance and sensitivity. Moreover, the electronic tag antenna and the reader-writer may not even work normally. Thus, a wave absorbing magnetic sheet may be attached onto the antenna to absorb lines of magnetic force close to the antenna without passing through the metal parts, so that eddy current losses may not be generated so as to avoid the shortening of the communication distance and the decrease in sensitivity. The wave absorbing magnetic sheet is different from other general wave-absorbing materials. General wave-absorbing materials absorb electromagnetic waves and make electromagnetic energy lost, which requires that the imaginary part of the permeability, i.e. the magnetic loss, is relatively large. However, wave absorbing magnetic sheets for NFC antennas just absorb electromagnetic waves to converge the lines of magnetic force without any loss of the electromagnetic energy, which requires that the real part of the permeability is as large as possible and the imaginary part of the permeability is as small as possible, i.e. the quality factor is high.
To meet the requirements of the flexibility and the miniaturization of electronic devices, the existing NFC composite magnetic sheets are made into thin sheets primarily by casting after magnetic powders are loaded by an organic carrier. The adjustment of the permeability of the magnetic sheet is mainly done on the magnetic powder, for example, by doping or heat treatment. However, when the magnetic powder is determined, it is very hard to increase the real part of the permeability and decrease the imaginary part of the permeability by improving the manufacturing process. For example, both the real part and the imaginary part of the permeability may increase when the density of the magnetic sheet increases, but the quality factor does not necessarily increase.